Advanced CMOS using super steep retrograde wells

ABSTRACT

The present invention is a method for forming super steep doping profiles in MOS transistor structures. The method comprises forming a carbon containing layer ( 110 ) beneath the gate dielectric ( 50 ) and source and drain regions ( 80 ) of a MOS transistor. The carbon containing layer ( 110 ) will prevent the diffusion of dopants into the region ( 40 ) directly beneath the gate dielectric layer ( 50 ).

FIELD OF THE INVENTION

The present invention relates to CMOS transistors formed using supersteep retrograde wells. The method of formation of the retrograde wellinvolves using a carbon doped capping layer. The method is applicable toboth P-Well and N-Well formation by altering the diffusioncharacteristics of dopants such as B, P, In, and As for optimizedretrograde well profile versus total thermal budget seen by these wellduring subsequent processing steeps.

BACKGROUND OF THE INVENTION

As advanced CMOS technology continues to scale and move into thedeep-sub-micron geometry dimensions for core devices, proper channelengineering of the CMOS devices becomes increasingly important. One ofthe more promising methods for extending the performance of CMOS devicesas technology continues to scale, is the incorporation of super steepretrograde wells and a thin intrinsic region for the channel of the CMOSdevices. In forming a retrograde well the dopant concentration inregions further from the gate dielectric of the transistor is higherthat that in regions adjacent to the transistor gate dielectric.

A typical MOS transistor is shown in FIG. 1. Isolation structures 20 areformed in the substrate 10. The gate dielectric layer 50, the conductivegate layer 60, and the sidewall structures 70 comprise the gate stack.In an enhancement mode transistor, the source and drain regions 80 areof an opposite conductivity type to that of the substrate region 10. Asdescribed above, in a retrograde well the dopant concentration in region30 is greater than that of the channel region 40, with a concentrationgradient that is typically limited by diffusion of the dopant species.In the ideal case what is required is a super steep dopant concentrationprofile from region 30 to region 40 with region 40 being intrinsicallydoped. The use of super steep retrograde wells with intrinsically dopedchannel regions has significant performance advantages for CMOS devices.These advantages include reduction of short channel effects, increasedmobility in the channel region, higher mobility, less parasiticcapacitance, and a reduction in short channel effects. Although thesuper steep retrograde wells have significant advantages for advancedCMOS devices, it is very difficult to achieve these structures whenmanufacturing these devices for high volume integrated circuitapplications. This difficulty is due to the out-diffusion of theretrograde well dopant species into the channel region especially forp-well device such as the NMOS transistor. In fact, it has been shownthat current silicon processing techniques will not be able to achievestringent doping profiles that are targeted to change by as much asthree orders of magnitude in less then 4 nm by the year 2008. There istherefore a great need for new processing techniques that will allow theformation of super steep retrograde well structures with near intrinsictransistor channel regions.

SUMMARY OF INVENTION

The improved MOS transistor of this type according to the presentinvention is characterized by the formation of carbon containing layers.The carbon containing layers will retard diffusion of the dopant speciesin the various regions of the MOS transistor. The methodology of thepresent invention offers many advantages over existing technology. Thepresent invention allows for a higher thermal budget in forming the SSRWof advanced CMOS technology which increases dopant activation and hencereduces the “on state” resistance. It is now possible to simultaneouslyachieve ultra shallow vertical source drain junction conditions and aSSRW. Improved analog matching in individual NMOS and PMOS transistorscan now be achieved since the fabrication process is now less sensitiveto thermal variations during rapid thermal annealing across the wafercompared to processes that have faster diffusion rates and hence areless well controlled. The technique can be used to balance arsenicdiffusion with boron diffusion for fabricating symmetric NMOS and PMOSdevices in a typical CMOS process flow. It can also be used to makeasymmetric source and drain structures by controlling carbonimplantation in the source and drain regions. This may have benefits formaking ESD and higher power devices in CMOS circuits. By using a mask topattern the carbon (or SiGeC) implants it is possible to make a higherthreshold and a low threshold voltage device while minimizing sourcedrain implant diffusion in the vertical direction. The methodologyallows boron to be extended into scaled deep submicron CMOStechnologies, which have had to shift to indium dopants to achieve SSRWand ultra shallow junctions. Although indium works in these technologiesit is non-standard for most CMOS processes and has a lower salabilitylimit and is susceptible to carrier freeze out effects at roomtemperature which limits its usefulness. Other technical advantages willbe readily apparent to one skilled in the art from the followingFIGUREs, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals represent like features, in which:

FIG. 1 is a cross-sectional diagram showing a typical MOS transistor.

FIGS. 2A-2B are cross-sectional diagrams showing an embodiment of supersteep retrograde well MOS transistor formed using ion implantation.

FIGS. 3A-3B are cross-sectional diagrams showing an embodiment of supersteep retrograde well MOS transistor formed using epitaxial growth.

FIGS. 4A-4B are cross-sectional diagrams showing an embodiment of supersteep retrograde well MOS transistor using carbon halo implants.

FIG. 5 is a cross-sectional diagram showing high voltage and low voltagetransistors formed using an embodiment of instant invention.

FIGS. 6A-6B are cross-sectional diagrams showing an embodiment of supersteep retrograde well MOS transistor using deep carbon pocket implants.

FIG. 7 is a cross-section diagram showing a further embodiment of theinstant invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described with reference to FIGS. 2-6. Itcomprises super steep retrograde well (SSRW) structures formed using acarbon based capping layer. In general the SSRW is very sensitive todiffusion of dopants and the total thermal budget that the SSRWencounters during processing. Transient enhanced diffusion (TED) and thehigh diffusion rate of boron makes it very difficult to maintainhyper-abrupt SSRW doping profiles during manufacturing. In the instantinvention a thin carbon containing film is used as a diffusionbarrier-capping layer over the SSRW. This capping layer can beepitaxially grown or implanted. A near intrinsic channel region can thenbe formed above the capping layer. Ideally this intrinsic channel regionis a thin layer that is typically 50 Å to 300 Å in thickness.

Shown in FIGS. 2( a) and 2(b) are the formation of a SSRW MOSFET usingion implantation. As shown in FIG. 2( a), a silicon substrate 10 isprovided and isolation structures 20 are formed in the substrate 10.These isolation structures consist of shallow trench isolation (STI) orLOCOS. In STI, trenches are formed in the substrate 10 which are thenfilled with a insulating dielectric. In an embodiment of the instantinvention, the dielectric that is used to fill the trench and form theisolation structure is a silicon oxide. In other embodiments siliconoxynitride or silicon nitride can also be used to form the isolationstructures 20. LOCOS isolation comprises masking regions of thesubstrate before performing thermal oxidation to form the localizedisolation structures. Following the formation of the isolationstructures, a masking layer 90 is formed to selectively mask the n-welland p-well regions during either well formation process. This maskinglayer usually comprises photoresist. Illustrated in FIGS. 2( a) and 2(b)will be the formation of a p-well which is used in the fabrication aNMOS transistor. A similar process can be used to form a n-well bysimply changing the implanted species from p-type to n-type. In forminga SSRW according to the instant invention a four implantation processwill be illustrated in FIG. 2( a). The instant invention is not howeverlimited to a four implant process. Any number of implant steps can beused to form the SSRW without exceeding the scope of the invention. In afour step process to form the p-type retrograde well four differentimplants of p-type dopants are performed. A deep high energy wellimplant is performed to form the deep p-well region 120 shown in FIG. 2(a). In an embodiment of the instant invention this well implant willcomprise dopants such as boron, or a boron containing species atenergies of 300-400 KeV and doses of 1×10¹³-1×10¹⁴ cm⁻². A channel stopimplant is performed to form the channel stop region 130 shown in FIG.2( a). In an embodiment of the instant invention this channel stopimplant will comprise dopants such as boron, or a boron containingspecies at energies of 100-200 KeV and doses of 1×10¹²-1×10¹³ cm⁻². Apunch through implant is performed to form the punch through region 140illustrated in FIG. 2( a). In an embodiment of the instant inventionthis punch through implant will comprise dopants such as boron, or aboron containing species at energies of 50-100 KeV and doses of1×10¹²-1×10¹³ cm⁻². These three implants will form a SSRW. In additionto boron other p-type dopants such as gallium and indium could also beused to form the retrograde p-well region. To prevent the diffusion ofthese species into the transistor channel region a carbon capping layeris formed beneath the transistor channel region. Such a carbon cappinglayer 110 is shown in FIG. 2( a). In the instant case this carboncapping layer 110 is formed by implanting carbon or a carbon containingspecies into the substrate 10. The conditions of the carbon implantshould be such that the capping layer has a carbon concentration ofgreater that about 0.1 atomic percent. The thickness of the cappinglayer should about 10-1000 angstroms and it should be positioned belowthe transistor channel region but above the peak of the punch throughimplant. A threshold implant can be performed to adjust the transistorthreshold voltage by forming the dopant region 150 shown in FIG. 2( a).In an embodiment of the instant invention this threshold voltage implantwill comprise dopants such as boron, or a boron containing species atenergies of 5-20 KeV and doses of 1×10¹²-1×10¹³ cm⁻².

Shown in FIG. 2( b) is a MOS transistor fabricated in the SSRW of FIG.2( a). The presence of the carbon layer 110 will prevent diffusion ofthe boron up through the SSRW and into the channel region 40 of thetransistor. In addition, if phosphorous is used in forming the sourceand drain regions 80 the capping layer 110 will prevent the diffusion ofthe phosphorous species into the well region resulting in the formationof shallow drain and source regions. The thickness of the source anddrain regions will be determined by the distance W (155 in FIG. 2( b))of the capping layer from the substrate surface under the gatedielectric layer 50. In an embodiment of the instant invention thedistance W is about 50 A to 800 A. The transistor structure shown inFIG. 2( b) comprising the gate dielectric layer 50, the conductive gatelayer 60, the sidewalls 70, and the source and drain regions 80 can befabricated using standard processing techniques. The application of theinstant invention to the formation of a n-well would simply involvechanging the species used for the well, channel stop, punch through, andthreshold voltage implants from p-type to n-type. Such n-type speciescould comprise arsenic, phosphorous, or antimony with energies and dosesof 500-600 KeV and 1×10¹³-1×10¹⁴ cm⁻², 300-400 KeV and 1×10¹²-1×10¹³cm⁻², 100-200 KeV and 1×10¹²-1×10¹³ cm⁻², and 5-50 KeV and 1×10¹²-1×10¹³cm⁻² respectively.

Shown in FIGS. 3( a) and 3(b) is a further embodiment of the instantinvention. In this embodiment the capping layer is formed usingdeposition processes illustrated in FIG. 3( a). Starting with thesilicon substrate 10, a carbon, carbon doped silicon layer, or asilicon-germanium-carbon (SiGeC) layer 160 is deposited on the surfaceof the substrate 10. The carbon concentration in the layer 160 must begreater than 0.1 atomic percent and the layer thickness should bebetween 10 and 1000 angstroms. Following the formation of the carboncontaining layer 160, a silicon epitaxial layer 170 is formed over thecarbon containing layer 160. As shown in FIG. 3( b), isolationstructures 20 are formed in the structure shown in FIG. 3( a) asdescribed above. A SSRW can be formed in the substrate 10 beneath thecarbon containing layer using ion implantation. The position of such aSSRW is indicated by 177 in FIG. 3( b). The various regions formed bythe ion implantation steps are omitted from the Figure for clarity. TheMOS transistor is then fabricated in the silicon epitaxial layer 170overlying the carbon containing layer 160. In addition, the depth of thesource and drain regions 80 will be determined by the thickness X (175in FIG. 3( b)) of the silicon epitaxial layer 170.

Shown in FIGS. 4( a) and 4(b) is a further embodiment of the instantinvention. As shown in FIG. 4( a), isolation structures 20 are formed ina silicon substrate 10. A gate dielectric layer 50 and a conductive gatelayer 60 is formed and patterned to define the gate structure (50 and60) shown in FIG. 4( a). The lightly doped drain and source extensionregions 180 are formed by performing a self-aligned implant with thegate structure (50,60). To reduce the gate length dependence oftransistor threshold voltage, angled halo implants are often performedto introduce dopants under the gate structure (50, 60). In the instantinvention, a layer of carbon 195 is first implanted under the gatestructure followed by the normal halo implantation process which resultsin region 190 being formed. As shown in FIG. 4( a), the carboncontaining layer 195 encapsulates the implanted halo region 190. It isrequired that the carbon concentration in layer 195 be greater than 0.1atomic percent to effective inhibit dopant diffusion. For a typical NMOStransistor such a halo implantation process might be boron speciesimplanted at energies of 5-50 KeV and doses of 1×10¹²-1×10¹³ cm⁻². Thecarbon layer 195 will prevent the diffusion of the boron species in thehalo region 190 from diffusing further under the gate. Following theformation of the halo regions 190 and the carbon regions 195, sidewallstructures 70 are formed followed by the formation of the source anddrain regions 200 by ion implantation. It should be noted that thestructure illustrated in FIG. 4( b) can be combined with any of the SSRWschemes described above to include a carbon containing layer beneath thesource drain region 200 and a SSRW beneath the carbon containing layer.

Shown in FIG. 5 is a further embodiment of the instant invention. Theembodiment describes the formation of a high threshold voltage device225 and a low threshold voltage device 215. Using the methodologydescribed above, multiple implantation processes are used to form SSRWs212 and 214 beneath both gate dielectric layers 50 and 55. The carboncontaining capping layer 210 beneath transistor 215 is contiguous whilethe carbon containing capping layer beneath transistor 225 is comprisedof two sections 220 separated by a gap. The gap between the sections 220is positioned beneath the conductive gate layer 62 which is formed onthe gate dielectric layer 50. This allows dopant species to diffuse upthrough the gap 230 and increase the dopant concentration in the channelregion 40 thereby increasing the threshold voltage of transistor 225.This is to be contrasted with transistor 215 where the capping layer 210prevents the diffusion of dopant species. The threshold voltage oftransistor 215 will therefore be lower than that of transistor 225 forthe same ion implantation conditions. The transistors 215 and 225 willalso have conductive gate layers 60 on the gate dielectric layers 50 aswell as sidewall structures 70 and 75 adjacent to the conductive gatelayers 60 and 62. The structure illustrated in FIG. 5 can be combinedwith any of the SSRW schemes described above.

Shown in FIGS. 6( a)-6(b) is a further embodiment of the instantinvention. In this embodiment a carbon containing capping layer 210 isformed using any of the methods described above. The layer 210 shouldhave a carbon concentration that is greater than 0.1 atomic percent witha thickness between 10-1000 angstroms. After formation of the gatedielectric layer 50 and the conductive gate layer 60, deep pocketimplants are performed to form the doped region 240 beneath the carboncontaining capping layer 210. The presence of the carbon capping layer210 will prevent the diffusion of the dopant species in region 240 upinto the channel region 40. Following the formation of the deep pocketregion 240, the sidewall structures 70, and the source and drain regions80 are formed using standard processing techniques. The completedtransistor structure is shown in FIG. 6( b). The structure illustratedin FIG. 6( b) can be combined with any of the SSRW schemes describedabove.

Shown in FIG. 7 is a further embodiment of the instant invention. Inthis embodiment a silicon on insulator (SOI) substrate is provided witha substrate 245, a buried oxide layer 250, and an upper silicon layer255 in which the MOS transistor is formed. The isolation structures 20are formed as described above. A carbon containing layer 260 is formedby implanting carbon or a carbon containing species asymmetrically inthe source and drain regions of the MOS transistor. The layer 260 shouldhave a carbon concentration that is greater than 0.1 atomic percent. Thegate dielectric layer 50, the gate layer 60, and the sidewalls 70 areformed as described above. The asymmetric regions 270 and 280 canfunction as either the source or drain of the transistor. The carboncontaining layer 260 will prevent the diffusion of the species used toform region 270 into region 290. Region 290 can therefore be used toprovide a substrate contact for the transistor.

The methodology of the instant invention offers many advantages overexisting technology. The instant invention allows for a higher thermalbudget in forming the SSRW of advanced CMOS technology which increasesdopant activation and hence reduces the “on state” resistance. It is nowpossible to simultaneously achieve ultra shallow vertical source drainjunction conditions and a SSRW. Improved analog matching in individualNMOS and PMOS transistors can now be achieved since the fabricationprocess is now less sensitive to thermal variations during rapid thermalannealing across the wafer compared to processes that have fasterdiffusion rates and hence are less well controlled. The technique can beused to balance arsenic diffusion with boron diffusion for fabricatingsymmetric NMOS and PMOS devices in a typical CMOS process flow. It canalso be used to make asymmetric source and drain structures bycontrolling carbon implantation in the source and drain regions. Thismay have benefits for making ESD and higher power devices in CMOScircuits. By using a mask to pattern the carbon (or SiGeC) implants itis possible to make a higher threshold and a low threshold voltagedevice while minimizing source drain implant diffusion in the verticaldirection. The methodology allows boron to be extended into scaled deepsubmicron CMOS technologies, which have had to shift to indium dopantsto achieve SSRW and ultra shallow junctions. Although indium works inthese technologies it is non-standard for most CMOS processes and has alower salability limit and is susceptible to carrier freeze out effectsat room temperature which limits its usefulness. While this inventionhas been described with reference to illustrative embodiments, thisdescription is not intended to be construed in a limiting sense. Variousmodifications and combinations of the illustrative embodiments, as wellas other embodiments of the invention will be apparent to personsskilled in the art upon reference to the description. It is thereforeintended that the appended claims encompass any such modifications orembodiments.

1. A method of forming a SSRW, comprising: providing a semiconductorsubstrate with an upper surface; forming a carbon containing layer onthe upper surface of the semiconductor substrate; forming a siliconlayer on the carbon containing layer; and performing a plurality ofimplants into the silicon substrate to form a retrograde well regionbeneath said carbon containing layer.
 2. The method of claim 1 whereinthe carbon containing layer comprises a silicon-germanium-carbon layer.3. The method of claim 1 wherein the carbon containing layer comprises acarbon doped silicon layer with a carbon concentration of greater than0.1 atomic percent.
 4. The method of claim 1 wherein the carboncontaining layer has a thickness of 10-1000 angstroms.